Add `nix.linux-builder.systems` option to set corresponding `nix.buildMachines.*.systems` option

[ethnt]

I've set up my local linux-builder to emulate x86_64-linux as well as aarch64-linux (jury's out as to how well that will work), but I still need to be able to set that option in /etc/nix/machines. I've added an option for this in nix.linux-builder. Potentially in the future it would be a better idea to simply allow extraBuilderConfig or something similar, but I stuck with convention in this PR.

[ethnt]

@Enzime Thanks for the review, I've made those changes!